Switching regulators are normally utilized to provide lightweight high efficiency regulated power supplies. In a basic switching regulator, a switching device is connected in series with a load, and regulation of the output is accomplished by on/off switching of the switching device through a feedback circuit. The feedback circuit samples the output voltage and compares it to a reference voltage. The difference (error signal) between the two voltages is used to control the on/off duty cycle of the switching device. If the output voltage tends to decrease below the reference voltage, the duration of the on time pulse increases. The switching device then conducts for a longer period of time so that the output voltage increases to the desired level. If the output voltage tends to ride above the reference voltage, the duration of the on time pulse decreases. The shorter conduction period of the switching device then results in a compensating difference in the output voltage. Some type of filter is required between the switching device and the load to obtain a smooth DC output. A commonly used filter consists of an LC network and a commutating diode.
An additional disadvantage of a switching regulator is that the capacitor in the output filter can charge to peak values that approach the level of the input voltage if the load current is low. This is usually resolved by adding a shunt load that may be either across the output all the time or just connected when the current in the load is very low. This affects the efficiency of the system because of undesirable dissipation in the shunt load.
The major advantage of the switching regulator over the linear regulator is the higher efficiency that results from the mode of operation of a series pass transistor utilized as the series switching device. In this mode of operation, the transistor is operated in its two most efficient stages, either at cutoff or at saturation. As a result, dissipation is considerably less than when the transistor is operated in the linear region. The response time of the switching regulator, however, is usually slower than that of the linear regulator, but can be improved by operation of this circuit at higher frequencies.
The disadvantage to the above described switching regulator is that the operating frequency is dependent upon the error voltage which may vary over a large range, thus generating an enormous amount of electromagnetic interference (EMI). Switching regulators are usually notorious for their EMI. To improve EMI immunity, methods have been employed such as maintaining a constant frequency into the switching regulator and varying the pulse width. Feedback schemes are dependent both upon their gain and bandwidth in determining their operation characteristics. Depending upon the load and the unregulated input voltage, regulation is determined by the response of the feedback. To correct for both input voltage variations and load changes, the feedback system must compensate by maintaining a high gain. As the frequency changes the feedback system must have a wide bandwidth to accommodate for rapid changes. This is a disadvantage in that high gain and wide bandwidth often result in inherent instabilities.
In view of the above problems, there exists a need for a switching regulator utilizing a switching control that reduces sensitivity to input voltage variations and load changes while maintaining a high gain and wide bandwidth.